The present invention relates generally to high rate packet data networks for mobile communications and, more particularly, for high rate packet data networks.
The Telecommunications Industry Association (TIA) recently adopted a new specification for high rate packet data services known as TIA/EIA/IS-856 “cdma2000, High Rate Packet Data Air Interface Specification.” Communication networks based on this standard can provide up to 2.4 Mbps in a standard bandwidth 1.25 MHz channel.
In HDR networks, the reverse link traffic channel between the access terminal and the access network is a variable rate channel. The access terminal selects the data rate from among one of five possible data rates and can change the data rate on a packet-by-packet basis without negotiation with the access network. Therefore, the amount of reverse link interference experienced by the base station receiver varies rapidly and by large amounts. A fast flow control method is therefore needed to prevent congestion on the reverse link communication channel.
The TIA/EIA/IS-856 standard describes a flow control method that uses the reverse activity (RA) channel to regulate traffic flow on the reverse link communication channel. More particularly, flow control is accomplished by transmitting a “busy” signal from the access network to the access terminals that throttles back the data rate permissible by the access terminals served by the base station. The busy signal is carried by the Reverse Activity (RA) channel, which is typically broadcast by each radio base station in the access network, where the coverage area of each radio base station defines a sector. Reverse link loading is estimated for a sector by measuring the total received signal power, which is accomplished by reading the control voltage of the base station receiver automatic gain control (AGC) module, and comparing it against the nominal receiver noise figure. The reverse link load estimate is then compared against an overload threshold to control the RA channel busy signal.
The AGC measurements required for flow control under the TIA/EIA/IS-856 standard are analog in nature and noisy. Therefore, AGC measurements must be low pass filtered in order to enhance their reliability. The time constant for low pass filtering is typically in the order of 1.28 seconds. Thus, low pass filtering introduces delay in the generation of reverse link loading estimates, which, in turn, increases the response time of the base station to changes in reverse link loading. The longer response time means that the base station may not be able to track fast changes in reverse link loading and may, therefore, be ineffective under such conditions.
Another disadvantage of the prior art flow control method is that reverse link load estimates based on AGC measurements and nominal receiver noise values are highly inaccurate. The receiver noise figure can be measured only in the factory during equipment calibration and varies with temperature and aging of components. So that the noise figure is used as a reference in the reverse link load estimation, any inaccuracy in the noise figure nominal value directly affects the accuracy of the reverse link load estimation.
Accordingly, an improved method and apparatus for flow control that is capable of tracking fast changes in reverse link loading are needed.